Tin of good quality should be white and lustrous and of crystalline structure
and, if in rods, it should give the characteristic crackling when bent in different
directions. It should not contain more than 0-1% of sulphur and arsenic
together, or more than 0-01% of bismuth, or more than i% of total impurities
The compositions of tins of different origin are as follows (Hollard, Schnabel) •'
Composition of Samples of Tin
1 ! Fe
Settlements- Malacca . (Pulo Brani .
China (Hong-Kong) England
~ (I ....
Germany -I1 TT
With commercial tin-plate the most important determination to be
made is that of the tin, the thickness of the layer of this metal determining
the value ; next comes the determination of the lead, which gives an indi-
cation of the quality of the tin used. For these two determinations see i.
Sometimes, however, for certain purposes (hygienic, for instance) deter-
mination of the lead alone is of interest, it being desired to ascertain if the
tinning has been done with commercial tin or with an alloy of lead and tin.
The determination of the lead alone is described in 2.
1. Determination of the Tin and Lead.—From i to 2 sq. dms. of
the tin-plate 1—taken from the middle of the plate, since the tinning is
often irregular at the edges—is measured exactly, de-fatted with ether or
benzene and weighed, so that the results may be referred to the weight as
well as the surface of the sample. The measured portion is cut into small
squares of 1-2 cm. side, these being heated to boiling for 5 minutes in a
covered beaker with 50 c.c. of 10% hydrochloric acid (25-30 c.c. cone,
hydrochloric acid diluted to 100 c.c. with water). The hydrochloric acid
solution is decanted into a 300 c.c. measuring flask and the metal boiled
for 5 minutes with another 50 c.c. of the acid, this being decanted off and,
if necessary, the residue treated a third time ; the colour of the metal should
be a uniform iron-grey when all the tin is dissolved. Tin and lead are
easily dissolved in the hot by hydrochloric acid of this concentration, while
the iron is attacked but little.2 The residue is washed with a little water
and this decanted into the 300 c.c. flask, the contents of which are rendered
1 With a very irregular sample a larger quantity is taken.
2 If the tin-plate is very rich in lead, a few drops of nitric acid are added to the
hydrochloric acid,d off on an asbestos filter (Gooch crucible),